1. Field of the Invention
The present invention relates to a fuel-cells system, an electric vehicle with a fuel-cells system, and a method of controlling a supply of electric power. More specifically the present invention pertains to a technique of keeping a remaining charge of a storage battery in a predetermined range in a fuel-cells system having a stack of fuel cells and the storage battery as power sources.
2. Description of the Prior Art
Proposed fuel-cells systems have a stack of fuel cells and a storage battery as power sources that supply electric power to a loading (for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 47-32321). In such a fuel-cells system, a predetermined contact disposed in a circuit constituting the fuel-cells system opens and closes, in order to enable a stack of fuel cells alone to supply electric power to a relatively small loading and enable both the stack of fuel cells and a storage battery to be discharged and supply electric power to a greater loading. In case that the remaining charge of the storage battery is lowered, the connection of the circuit is switched to enable the fuel cells to charge the storage battery while supplying electric power to the loading. The fuel cells are characterized by a voltage decrease with an increase in output electric current. This proposed structure having both the fuel cells and the storage battery ensures a sufficient output even when the loading has large power consumption.
In the proposed fuel-cells system however, when the remaining charge of the storage battery is lowered, the predetermined contact disposed in the circuit constituting the fuel-cells system is switched to change the state of the storage battery from the discharging condition to the charging condition. Upon condition that a large amount of electric power is supplied to the loading, a decrease in remaining charge of the storage battery results in switching the contact. In case that the loading continuously requires large power consumption, no supply of electric power from the storage battery causes the fuel cells to be exposed to the excess loading. This significantly lowers the output voltage and thereby causes an insufficient output.
The above structure controls the charge-discharge state of the storage battery by opening and closing the predetermined contact according to the remaining charge of the storage battery. Another possible structure has the storage battery and the fuel cells arranged in parallel to each other and changes the output state of the storage battery between the charging state and the discharging state, based on the output voltage difference between the storage battery and the fuel cells that varies with a variation in amount of loading. In the latter structure, the remaining charge of the storage battery is continuously lowered under the condition of large loading, and the fuel cells are accordingly exposed to excess loading.
In these structures, in case that the lowered remaining charge of the storage battery results in an insufficient supply of electric power from the storage battery, an increase in loading applied to the fuel cells may lead to various problems. When an increase in loading to the fuel cells causes the output electric current from the fuel cells to exceed a predetermined level, increase in supplies of material gases fed to the fuel cells does not sufficiently increase the amount of power generation. This also makes the voltage unstable and may cause fan abrupt drop of the voltage.
When the output electric current from the fuel cells exceeds the predetermined level and the voltage is made unstable, a phenomenon that reverses the positive electrode and the negative electrode and is generally referred to as the change of poles may be observed in a unit cell included in the fuel cells. The change of poles makes the voltage unstable and causes the energy, which is expected to be converted to electrical energy through electrochemical reactions, to be released as thermal energy. The release of thermal energy causes partial excess heating in the fuel cells and thereby shortens the life of the fuel cells.